1. Field of the Invention
The present invention relates to an electronic flash device arranged to prevent a main capacitor thereof from being overcharged.
2. Description of Related Art
According to the conventional arrangement of an electronic flash device, when a main capacitor happens to be overcharged beyond a charging completion voltage because of an abnormal operation occurring by some reason (for example, in a case where a control signal line is disconnected by external impact), the main capacitor which is an aluminum electrolytic capacitor generates a gas, which then causes an explosion proof valve to act to release the gas to ensure no further trouble.
However, the generation of gas tends to give a disagreeable feeling to the operator. Besides, in some cases, the operator might use the flash device without noticing a malfunction resulting from overcharging. In view of this, it has been developed to cause the main capacitor to discharge its electric charge by forcibly causing light emission when the main capacitor is overcharged, i.e., when the voltage of the main capacitor reaches a rated voltage thereof. To detect such an overcharged state, a series circuit composed of a Zener diode and a resistor is connected in parallel with the main capacitor and the gate of a trigger thyristor is connected to the cathode side of the Zener diode in such a way as to forcibly trigger the light emission when the electric charge of the main capacitor reaches the rated voltage.
FIG. 8 shows by way of example the conventional circuit arrangement of a flash device. Referring to FIG. 8, a boosting circuit 2000 is connected to a battery 1000 to boost a battery voltage. A voltage detecting circuit 3000 is arranged as a voltage detecting means for detecting the charging voltage of a main capacitor 4000. A forcible light emission circuit 5000 is arranged to forcibly cause light emission when the voltage of the main capacitor 4000 reaches a rated voltage. A Zener diode 5001 has its cathode connected to the anode of the main capacitor 4000 and its anode connected to one end of a resistor 5002. The other end of the resistor 5002 is connected to a gate of a thyristor 7000 and one end of a resistor 5003. The other end of the resistor 5003 is connected to a cathode of the main capacitor 4000. The other end of the resistor 5002 is also connected to one end of a resistor 11000. The other end of the resistor 11000 is connected to a control circuit 12000.
A resistor 6000 has its one end connected to the anode of the main capacitor 4000 and the other end to the anode of the thyristor 7000. The thyristor 7000 is provided for light emission and has its cathode connected to the cathode of the main capacitor 4000. To the anode of the thyristor 7000 is connected one end of a trigger capacitor 8000. The other end of the trigger capacitor 8000 is connected to the primary side of a trigger transformer 9000. A xenon tube 10000 is connected to the two ends of the main capacitor 4000. The control circuit 12000 includes a microcomputer and is arranged to control the input and output of an oscillation start signal C1, a charging voltage detection signal C2 and a trigger signal C3. A flash-device charging start switch 13000 is connected to the control circuit 12000.
The conventional circuit arrangement described above operates as follows. When the flash-device charging start switch 13000 is turned on, the oscillation start signal C1 turns on to cause the boosting circuit 2000 to perform a voltage boosting action for charging the main capacitor 4000. The voltage detecting circuit 3000 then detects the charging voltage of the main capacitor 4000 and generates the charging voltage detection signal C2. Then, the control circuit 12000 confirms completion of charging through the charging voltage detection signal C2. Upon confirmation of the completion of charging, the control circuit 12000 turns off the oscillation start signal C1 to bring the charging process to a stop. In the event of detachment of the terminal of the charge voltage detection signal C2, however, the charging process does not come to a stop. Under such a condition, when the charging voltage comes near to the rated voltage, the Zener diode 5001 which is set at a voltage level lower than the rated voltage turns on to turn on the gate of the thyristor 7000 through the resistor 5002. With the gate of the thyristor 7000 turned on, the electric charge of the trigger capacitor 8000 is discharged to generate pulses. Then, secondary pulses are generated by the trigger transformer 9000 to trigger and forcibly cause the xenon tube 10000 to emit light.